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Basic Communication Concepts

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Title: Basic Communication Concepts


1
Basic Communication Concepts
  • Rong Wang
  • Spring 2004

2
Recommended Reading
  • Chapter 3 of Data Communications From Basics to
    Broadband, 3rd Edition by William J. Beyda (ISBN
    0-13-096139-6)

3
Review of Data Communication
  • What is Communication?
  • What is Data Communication?
  • What is basic components of a Data Communication
    System?
  • What is Data?

4
Data Representation
  • Information comes in different forms
  • Text, numbers, images ,audio, video, etc.
  • With few exceptions, digital computers
    communicate through a series of 1s and 0s known
    as bits.
  • This binary representation can also be thought of
    as being on and off.
  • Groups of bits are referred to as bytes
  • In most systems, a byte consists of 8 bits
  • Usually each byte represents a single character
  • A-Z, a-z, 0-9
  • punctuation characters(e.g., _at_, , )
  • special characters (LF, CR, ESC)
  • Bits and bytes are closely related to the binary
    number system. See Appendix in text for more
    information

5
Character Codes
  • The relationship of bytes to characters is
    determined by a character code
  • Each time a user presses a key on a terminal/PC,
    a binary code is generated for the corresponding
    character.
  • Various character codes have been used in data
    communication including
  • Morse, Baudot
  • EBCDIC, ASCII
  • Unicode
  • Regardless of the character code, both the
    terminal/ host or sender/receiver must recognize
    the same coding scheme

6
Morse Code
  • First character code developed
  • For transmitting data over telegraph wires
  • telegrams (remember Western Union)
  • Used dots (short beep) and dashes (long beeps)
    instead of 1s and 0s
  • More frequent the character, the fewer the beeps

7
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8
Morse Code (contd)
  • Problems
  • variable length character representation
  • required pauses between letters
  • no lower case, few punctuation or special
    characters
  • no error detection mechanism

9
Baudot Code
  • One of first codes developed for machine to
    machine communication
  • Uses 1s and 0s instead of dots and dashes
  • For transmitting telex messages (punch tape)
  • Fixed character length (5-bits)
  • 32 different codes
  • increased capacity by using two codes for
    shifting
  • 11111 (32) Shift to Lower (letters)
  • 11011 (27) Shift to Upper (digits, punctuation)
  • 4 special codes for SP, CR, LF blank
  • Total 26 26 4 56 different characters

10
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11
Badout Code
  • Problems
  • required shift code to switch between character
    sets
  • no lower case, few special characters
  • no error detection mechanism
  • characters not ordered by binary value
  • designed for transmitting data, not for data
    processing
  • International Baudot
  • Added a 6th bit for parity
  • Used to detect errors within a single character

12
EBCDIC
  • Extended Binary Coded Decimal Interchange Code
  • 8-bit character code developed by IBM
  • used for data communication, processing and
    storage
  • extended earlier proprietary 6-bit BCD code
  • designed for backward compatibility or marketing?
  • still in use today on some mainframes and legacy
    systems.
  • Allows for 256 different character
    representations (28)
  • includes upper and lower case
  • lots of special characters (non-printable)
  • lots of blank (non-used codes)
  • assigned to international characters in various
    versions
  • used with/without parity (block transmissions)

13
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14
ASCII Code
  • American Standard Code for Information
    Interchange
  • 7-bit code developed by the American National
    Standards Institute (ANSI)
  • most popular data communication character code
    today
  • Allows for 128 different character
    representations (27)
  • includes upper and lower case
  • lots of special characters (non-printable)
  • generally used with an added parity bit
  • better binary ordering of characters than EBCDIC
  • Extended ASCII uses 8 data bits and no parity
  • Used for processing and storage of data
  • Allows for international characters
  • 8th bit stripped of for transmission of standard
    character set

15
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16
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17
UNICODE
  • Designed to support international languages
  • Latin Greek Cyrillic Armenian Hebrew
    Arabic Syriac Thaana Devanagari Bengali
    Gurmukhi Oriya Tamil Telegu Kannada
    Malayalam Sinhala Thai Lao Tibetan Myanmar
    Georgian Hangul Ethiopic Cherokee
    Canadian-Aboriginal Syllabics Ogham Runic
    Khmer Mongolian Han (Japanese, Chinese, Korean
    ideographs) Hiragana Katakana Bopomofo and Yi
  • Uses a 16-bit code for total of 65,536 possible
    char.
  • Incorporates ASCII in first 128 codes
  • Incorporates LATIN in first 256 codes
  • Support found in newer hardware software,
    especially web technologies (e.g., JAVA, XML,
    HTML)
  • For more see www.unicode.org

18
Summary of Character Codes
  • Morse .-
  • Baudot 5 bit (no parity)
  • Int. Baudot 6 bit (5 data 1 parity)
  • ASCII 8 bit (7 data 1 parity)
  • or
  • 8 bit (no parity)
  • EBCDIC 9 bit (8 data 1 parity)
  • or
  • 8 bit (no parity)
  • UNICODE 16 bits (no parity)
  • Normally terminals and hosts must use the same
    code
  • However, code conversion hardware/software can be
    used to allow different machines to communicate

19
Summary of Character Codes
  • Bits per character affect
  • storage requirements
  • throughput of information
  • Use of larger codes became feasible due to
  • higher transmission speeds
  • denser storage mediums
  • Choice of character coding scheme is a trade off
    between
  • simplicity brevity
  • expressivity

20
Transmission Characteristics
  • A character code determines what bits we will
    send between a terminal and host
  • But how will those bits be sent
  • Direction of Transmission Path
  • Parallel vs. Serial Transmission
  • Serial Transmission Timing
  • Line Topology
  • Others which well look at later
  • speed
  • organization of data (protocol)
  • transmission media

21
Flow of Transmission Path
  • Simplex
  • Half duplex
  • Full duplex

22
Simplex
23
Half Duplex
24
Full Duplex
25
Transmission Mode
26
Parallel Transmission
27
Serial Transmission
28
Note
In asynchronous transmission, we send 1 start bit
(0) at the beginning and 1 or more stop bits (1s)
at the end of each byte. There may be a gap
between each byte.
29
Figure 4.27 Asynchronous transmission
30
Note
Asynchronous here means asynchronous at the byte
level, but the bits are still synchronized
their durations are the same.
31
Note
In synchronous transmission, we send bits one
after another without start/stop bits or gaps.
It is the responsibility of the receiver to
group the bits.
32
Figure 4.28 Synchronous transmission
33
Efficiency Overhead
34
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